Heat Tracing and Hazardous Area

[pristava]

Is there any good article describing Heat Trace and Hazardous Area ?
I have a two softwares from Heat Trace Manufacturers, but I didn,t find that Ignitition Temperature of the Flammable Gas and Hot Surface Temperature of the Trace Heater are compared in these calculations. In my opinion the Surface Temperature of the Heater must be limited by the Ignitition Temperature of the Gas.

 

[rmw]

My experience with heat tracing was that only steam tracing was used in classified areas, but that was over a dozen years ago, and things might have changed by now.

rmw

 

[DRWeig]

RMW has the best solution -- avoid electric.

If you must use electric trace, there are NEC considerations. If your area is classified Div 2 (Zone 2), you may be able to get away with it (check with AHJ) as long as the thermostat is properly protected (it will spark in normal operation, so needs Div 1 (Zone 0,1) rating.

If your area is Div 1 (Zone 0,1), I don't think there's any way to get away with electric trace. All components would have to be listed and labeled for the atmosphere and I've never seen any.

The surface temp of the cable should be available from the manufacturers -- and you can look up the AIT for your particular gas in an MSDS sheet. I'd really advise using steam though. A little steam generator located back in the safe area would be the best....

Good luck!

Old Dave

 

[cuky2000]

A mixture of hazardous gases and air may ignite in contact with a hot surface. The condition for ignition depends on several factors as surface area, temperature and concentration of gas.
Equipment approved receives a temperature code indicating the maximum surface temperature of the equipment.
“Flammable gases and vapors are placed into NEC Groups based on a determination of explosion
pressures and maximum safe clearance between parts of a clamped joint under several
conditions. See NEC Article 500–505 for details. The various groups (A, B, C, and D for flammable
gases and vapors, E, F, and G for combustible dusts) are described in Article 500 of
the NEC”. (See table below)

_{NOTES:
Equipment that not exceed a maximum surface temperature of 212 oF (104 oF ambient temperature) is not required to be marked with a temperature code (NEC).
In addition to NEC check the following references: NFPA-496, IEEE Std 515, ANSI/ISA 12.12, UL 1604, Factory Mutual, NEMA 9.}

http://www.fmglobal.com/pdfs/P0296HazardousLocations2.PDF

 

[resqcapt19]

At least one manufacturer of electric heat trace says you can use it in Class I, Division I locations. The following is from their design and intallation guide.
"FOR FM APPROVAL, DIVISION 1 HAZARDOUS LOCATIONS
• Select material with the lowest AIT in F/°C.
• Multiply the ignition temperature by 0.8.
This temperature is the maximum allowable heating cable sheath temperature."

Don

 

[rbalex]

The general "rules" as they apply to US domestic work are found in NEC Sec 501.10 and a bit more specifically in 501.10(B)(1).

I refer you to the site below to get a feel for the various types involved: self-regulating, constant watt and mineral insulated (MI) cables.

http://www.nelsonheaters.com/Products/Products.htm

The site has some other reference material you may find addresses your specific request.

In Division 1, the entire system must be "identified" [501.10(A)]. Effectively, only MI cables can be used from restrictions included in 501.4(A) and temperature limitations/controls are part of the "identification" process; typically this is nationally recognized testing laboratory (NRTL) like UL or FM listing/labeling. (Note" listing/labeling does not necessarily mean NRTL certification, but you'll have a hard time "identifying" the equipment otherwise.) There is a general rule in the product standards (UL) that will limit exposed surface temperatures to 80 percent of the AIT.


Div 2 has a fairly broad range of acceptable solutions, effectively including general-purpose heaters with appropriate controls.

 

[rbalex]

I should have said,' "identified" is not necessarily NRTL certification...'

 

[rbalex]

http://www.nelsonheaters.com/DownLoad/Check_List_Forms/Checklists_and_Forms.htm

I guess I should correct myself again; apparently Nelson's self-limiting cables have Div 1 applications too.

IMO, steam is better for "bulk" applications, electric is best where close temperature control is necessary.

 

[TMD]

Raychem (Tyco) have lots of info on electric heat tracing.

http://www.tycothermal.com/europe/english/heating/supportmaterials/literature_index.asp